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Beware the Stagnation Zone in Your Pond


Updated: Sun, 14 Jun 2015

Over time, gravel at the bottom of the pond will clog with leaves and other debris that is blown into the pond, as well as 100% of the fish waste. Clogged filters force water to "channel" around the media instead of through it. Channeling water is then not filtered. Bacteria need food and oxygen to survive - with water channeling around the media - the bacteria die off. Dead bacteria and decaying organics in the biological filter become a haven for parasites and other diseases that can kill your fish!
Beware the Stagnation Zone in Your Pond

here were many designs that everyone thought was the mouse trap to beat all other mouse traps.......Up flow grids, down flow grids were one most common way it was done. Either way they were traps for all kinds of detritus, down flow over time kept stacking debris on to debris layer after layer. The break down is slower then what was collecting.

We found 3x3 foot square pits with a pump under (down flow)the grid supporting lava rock to pea gravel even up to 1/2 to 1" size rock pulling water down through media and then returning to waterfall outlet. They were even setup with external pumps as well. Now this works for a while with a VERY light bio load, but when you get leaves,air borne debris,fish poop uneaten food settling down loading up the whole concept changes and rather quickly. The first thing is channeling of media which now starts to go anaerobic(Lack of oxygen) after a few months you can see the water changing. You can see the Koi acting different as well,flashing,sitting on bottom,even red spots appearing on skin.

The other popular design was the up flow bio pits, this was designed to have a submersible or external pump pull from the bottom of pond. Then in turn return it into lower chamber under a grid supporting lava rock to pea gravel even up to 1/2 to 1" size rock (up flow) then flow over divider wall.Even for the filter pit to be under water flowing up back into main water column. The biggest problem with any of these designs was "How do you clean them"? Backwash? Flush media? And ones with pump(s)under media and grid how to replace or repair pump? with out having to drain entire pond to do so. Remember this was back in the mid to late 80's that this was the common filter. I have seen only a few that have the up flow filters pits that are working today. They have been upgraded with pre-filters,cartridges, which helps cut down on debris from collecting and also have back-washing ability's.

It is now the early part 2000's (2015) and we are seeing the come back of the UFG........Is this Good? or Bad? We won't be attempting any time soon the "New School" approach to UGF's as we had are fair share of them back in the late 80's to early 90's......they were also considered the "New School" approach back then as well. We will give a fair time frame to see where and how they do....... Its still early on in the game.

A few reference pictures:


No way to back wash system.




This concept back in 1995 was a vast improvement from the prior design.





This was another design, stilled needed high maintenance.

I want to re post an article from a gentleman I was able to spend sometime with and learn from one of the old school teachers.

Beware the Stagnation Zone in Your Pond
Gravel and Rock Form Dead Zone

by Joseph F. Cuny

It has been brought to my attention that some pond builders (both Koi and water garden) apparently are not familiar with basic pond design. In particular, I am referring to the use of gravel and rock inside the pond. As anyone who has been a pond keeper for a few years knows, that is a real no no. Possibly this practice is a direct carryover from the aquarium 'under gravel' filters, but it is a wrong application of the concept.

The problem is that the gravel and rock form a dead zone, more appropriately called a stagnation zone. Whether the pond has fish in it or only plants, there are all kinds of organic material produced in the pond. This organic material gets trapped in the stagnation zone and guess what happens? It stagnates! Since the water does not circulate very well in this zone, the water is deficient in dissolved oxygen, and the organics are processed anaerobically. In other words, the organic material rots or putrefies.

The end result of such putrefaction is the production of noxious gases and disease organisms. I do not know what effect these would have on plants but they are deadly on fish and are very unaesthetic. In time such a pond would smell like a cesspool! This is exactly what happens in an aquarium if the under gravel filter is not cleaned regularly. Even with oxygenated water flowing through such a filter, there are dead spots where the water does not flow and these have to be cleaned, usually with some type of vacuum or siphon system.

If it is necessary to put rocks on the bottom of a pond, possibly to simulate a natural stream, they should be well separated and bedded in mortar to allow flow around them and to eliminate pockets where debris could accumulate. The amount of work necessary to properly imbed gravel probably eliminates the use of gravel. If it is thought that the gravel will function as a filter, it should be placed such that it can be fairly easily cleaned. Despite claims made by various people, all filters must be cleaned. The only possible exception is the trickling filter type that is self cleaning, but in the process dumps the waste into the following stage where it must be captured and disposed of.

I have seen gravel filters built into a sump in the bottom of a pond. Such a filter is almost impossible to clean, and the resulting pond does not provide a healthy environment for the fish. With such a filter in a water garden, I would expect the same type of problems. I realize that many people believe that the root system of the plants provides space for the organisms that degrade the organic material, but this is not a suitable filter except for those organics that are soluble. Even these, when aerobically digested, result in material that falls to the bottom where it joins the leaves, stems, decaying roots, and so forth and then putrefies.

Joe Cuny is a founding editor and contributes to KOI USA since its beginning, over 20 years ago.
This article first appeared in the March/April 1999 KOI USA. It is reprinted here with permission.


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